1. Field of the Invention
This invention relates to semiconductor devices. More particularly, this invention relates to lead frames for mounting semiconductor chips that reduce the likelihood of bonding failures during fabrication of the devices.
2. Description of the Prior Art
Lead frames for semiconductor devices typically comprise a tab for mounting a semiconductor chip and a plurality of inner leads extending inwardly from the perimeter of the frame to an area immediately adjacent to the perimeter of the tab. The tab is typically located within the body of the lead frame and is secured to the perimeter of the frame by at least one supporting member. During fabrication of a semiconductor device the tips of the inner leads are connected to electrodes on the semiconductor chip by means of fine metal wires, following which the chip, tab and inner leads are encapsulated within a synthetic resin, a ceramic material, or a combination of a glass and a ceramic material. Semiconductor devices that can be fabricated in this manner include various types of integrated circuits.
Prior art lead frames suffer from a number of shortcomings. As the number of inner leads has been increased from 16 to 32 to 64, the width of these leads has usually been reduced. As the circumference of the lead frame is increased, the inner leads are correspondingly lengthened. These modifications have caused the inner leads to become weaker and easier to deform, to the extent that they can be bent by the vibrations experienced during transport of the lead frame. This problem has been further compounded by a reduction in lead frame thickness. A bent or dislocated inner lead increases the difficulty of bonding the lead to the find bonding wire connecting the lead to an electrode on the semiconductor chip, resulting in more frequent bonding failures and undesirable contact between adjacent bonding wires and/or inner leads.
Solutions that have been proposed to remedy the aforementioned shortcomings include stabilizing the configuration of the inner leads by adhering at least a portion of the leads to a strip of pressure-sensitive dielectric tape or adhering an edge of the mounting tab and the tips of at least some of the inner leads to a dielectric film adhered using a liquid adhesive. These solutions have been proposed in Japanese Patent Publications (Kokai) Nos. 58(83)/161,439, 58(83)/143,541 and 58(83)/182,859.
Use of a dielectric pressure-sensitive tape to immobilize the inner leads is less than satisfactory because the pressure-sensitive adhesive is softened by the high temperatures used in die bonding or wire bonding. The tape then slips or peels with the result that wire bonding to the leads is made difficult.
Lead frames in which the inner leads and tab surface are bonded to an insulating film using a liquid adhesive are less than satisfactory. The adhesive can be squeezed out during the bonding operation, thereby contaminating the wire bond and complicating the production process.
An objective of the present invention is to eliminate the problems associated with prior art lead frames by providing a lead frame for semiconductor devices in which the inner leads are immobilized, the wire bonding operation is simplified and failures to properly connect the semiconductor to the inner leads are infrequent.